The long-term objective of the proposed research is to understand the role of cell-cell communication in the maintenance and control of stem cell populations. As stem cells are of central importance in human development, and are of potential use in disease treatments, understanding how they work is an important public health goal. The organism chosen for the experiments is the flowering plant Arabidopsis thaliana, and the stem cell population the shoot apical meristem, which forms in embryos and is the source of all of the above-ground parts of the plant, including stem, leaves, and flowers. This plant has been chosen because its stem cells are particularly amenable to experiment - not only is there a great deal of background information on their patterns of gene activity, and on the mechanisms by which they communicate with each other, but also plant stem cells are at the plant surface, which allows them to be observed while alive and functioning without any invasive methods. The experiments proposed are designed to reveal the mechanism by which one set of plant stem cells communicates with another. It is known that the first set, the central zone cells at the top of the meristem, secrete a small protein that activates a receptor protein in the cells under them (the rib meristem cells). Preliminary results have revealed that the stability and cellular location of the receptor protein are determined by the small signaling protein. The proposed experiments examine the exact mechanisms by which the small signaling protein regulates the level and location of its own receptor in nearby cells. The methods to be used include live imaging of the cells and proteins involved, isolation of the genes and proteins involved, and also genetic screens for additional proteins that work along with the signal and receptor to control stem cell behavior. PUBLIC HEALTH RELEVANCE: A fundamental understanding of stem cell populations in plants will reveal basic principles of development and stem cell behavior, and will therefore eventually enable tissue engineering, and the use of genetically matched self-cells as a source of needed cell types and bodily organs to cure diseases and degenerative conditions. Plant development itself is also of direct health significance, as plants are the source of more than 80% of human food. A profound understanding of plant growth and development is a key to our ability to grow sufficient and nutritious food for the world in the future.